Polyploid Fragaria plants do not breed true and the genetics is very complex. The progeny of a vegetatively propagated variety fertilised with its own pollen are all different.
There have been some attempts to improve breeding strategies with a view to acquiring improved strawberry types however such strategies have met with limited or no success since problems associated with inbreeding depression have not been overcome.
Niemirowicz-Szycytt K.[(1989) Acta Horticulturae 265: 97-104] failed to control inbreeding depression within the inbreeding program employed and as a result did not obtain viable and suitably homogeneous (uniform) inbred lines capable of being used in inbreeding programs for obtaining agronomically attractive strawberry plants in commercial numbers.
Other workers have attempted to use alternative routes to obtain viable octoploid lines for use in strawberry breeding. Sayegh A. J. and Hernnarty M. J.[(1989) Acta Horticulturae 265: 129-135], attempted the production of haploids of commercial strawberry cultivars using androgenesis and intergeneric crosses followed by embryo rescue and culture. However, that approach was unsuccessful since the first essential step, that of obtaining haploids, was not realised and as a consequence the idea of doubling the haploid number to create octoploids and then selecting viable octoploid lines suitable for use in a breeding program could not be attempted.
A recurring problem associated with strawberry breeding programs as disclosed in the art is that inbred lines obtained through repeated selfing programs lack plant vigour and as a consequence are too weak to continue with further. Lack of plant vigour is generally evident at about the third or fourth or later generations.
In all cases, such programs have not been designed with a view to obtaining commercial quantities of F1 hybrids and seeds therefor. The goals of such breeding programs have been the acquisition of new breeding lines for use as basis for acquiring new varieties suitable for creating vegetatively propagated cultivars.
As a consequence of the difficulties associated with classical strawberry breeding techniques, breeders have faced an uphill task in improving their gene pool and each year breeders search for seedlings from within their breeding programs for plants which display desirable characteristics. Such Fragaria seedlings are propagated vegetatively by runners or by micro-propagation techniques for as long as necessary, however, seed from such seedling plants cannot generally be used for propagative purposes since plants resulting from such seed generally do not display phenotypic similarity in terms of commercially desirable characteristics in plant habit (Guttridge C. G. & Simpson D. W. Grower, Dec. 23,(1982) pp 28-29). Plants grown from seed harvested from vegetatively propagated cultivars typically display marked differences in appearance or plant habit. Such differences render them commercially and agronomically unacceptable.
Other disadvantages of supplying material grown via vegetative propagation and clonal propagation relate to the timing of release of such material to the grower. The propagator generally faces difficulties in timing and co-ordinating the release of sufficient numbers of strawberry plants to meet the requirements of the grower.
A further disadvantage of vegetatively propagated material is that there is a high risk of disease transmission in such material and when disease infestation is found, the use of chemicals to combat any disease infestation may become necessary which in turn not only leads to increased costs but also to potentially undesirable environmental effects. It follows from the above that if uniform inheritance of commercially desirable characteristics could be achieved in cultivated strawberry plants propagatable from seed such an achievement would mark a significant departure from current commercial strawberry production practices with the inbuilt limitations as outlined above, and pave the way for a highly attractive means of producing large numbers of strawberries in an efficient and cost effective manner.
Wild Fragaria species propagate from seed. However wild species generally produce small fruits and lack uniformity in appearance or plant habit.
Recently, there have been attempts to develop cultivated strawberry plants propagatable from seed, however, such attempts have met with limited success from a commercial point of view. One attempt at a seed propagatable strawberry, described as being commercially available from Pan American Seed is described in product brochures as seed propagatable, however, this variety suffers from the fact that up to only 75% of the plants grown from seed are capable of flowering and beating fruit in their first season. The brochure does not disclose how said variety was obtained. Whilst a seed propagated strawberry which performed to such a specification may have satisfactory value to a gardener it would have dubious worth from an agronomical point of view for the commercial grower.
High ploidy levels, such as octoploidy, in vegetatively propagated Fragaria, are generally associated with large berry size, while low ploidy levels such as diploidy are generally associated with smaller berry size. An example of a diploid species of strawberry, open pollinated F. vesca, is grown commercially from seed and is suitable for use in fruit drink, preserves production and the like, since the fruits which this species produces are small and not generally attractive for consumption fresh. The strawberry fruit production market is generally dominated by polyploid Fragaria species which are propagated via vegetative propagation. Such types include inter alia the octoploid species F. x ananassa. It would be highly desirable if polyploid Fragaria plants displaying commercially important characteristics could be propagated from seed reliably and without the need for vegetative propagation techniques.
It results from the above that it is highly desirable to provide seed giving rise to polyploid Fragaria plants of which substantially each plant is capable of fruit productivity within its first year from sowing. The seeds should conveniently give rise to plants displaying similarity (i.e. substantial uniformity) in one or more commercially valuable traits and where desired display similarity in its overall plant habit.
Despite such clear advantages of polyploid Fragaria plants that are propagatable by seed and of which substantially each plant is capable of fruit productivity, such plants/seed have not been provided for, the reasons being i.a. the complex genetics of the plants and the problems associated with the inbreeding depression typical for polyploid Fragaria plants.
It has now surprisingly been found that it is possible to apply inbreeding techniques suitable for diploids, such as the use of sib or half-sib families in inbreeding selection steps, to polyploid Fragaria plants and that when crossing a polyploid Fragaria plant having one desired characteristic (or trait) with another polyploid Fragaria plant, such characteristic will be expressed in substantially all of its progeny (i.e. within the bounds of biological certainty, which is, in general in more than 95% of its progeny).